There's been a lot of talk about AT&T's crusade against rogue tethering lately, and it all comes back to this AT&T text message sent to some poor iPhone customer(s):
AT&T Free Msg: We’ve noticed you’re continuing to enjoy the tethering feature with your smartphone service. Remember, you need a tethering plan ($45/mo, incl. 4GB) to use this feature, so we’re planning to update your line with the required plan soon.
Yes, the carrier everyone loves to hate is cracking down on unauthorized tethering - and they are automatically switching violators onto AT&T's 4GB data plan with tethering access, at a steep $45 a month.